1. Field of the Invention
The present invention generally relates to an apparatus and a method of distortion compensation for a power amplifier, and especially relates to the apparatus and the method of distortion compensation for the power amplifier used by radio base stations of a cellular phone system, digital broadcasting, and the like.
2. Description of the Related Art
As a technique for reducing distortion of a transmission power amplifier used by radio base stations of a cellular phone system, digital broadcasting, and the like, a pre-distorter has been used. For example, a pre-distorter proposed by JP,54-140856,A suppresses distortion of a power amplifier by applying a reverse distortion coefficient to the power amplifier according to envelope amplitude of a source signal to be transmitted. Characteristics of the reverse distortion are determined by obtaining a difference between the source signal to be transmitted and the signal output by the power amplifier, and are determined such that the difference becomes zero, using a method such as LMS (Least Mean Square).
A pre-distorter proposed by Japanese patent application 2001-512717 includes, as shown in FIG. 1, an envelope amplitude calculation unit 10 for acquiring envelope amplitude of an incoming signal, an envelope time difference calculation unit 12 for calculating time difference of the envelope amplitude of the incoming signal, a reverse distortion coefficient table 14 that is searched to obtain a reverse distortion coefficient corresponding to the envelope amplitude and the time difference. Further, complex multiplication of the incoming signal by the reverse distortion coefficient is performed by a multiplication unit 16, the result of which is amplified by a power amplifier 18, and an output signal is transmitted.
Further, a part of the output signal of the power amplifier 18 is branched by a directional coupler 20, and is compared with the incoming signal before distortion compensation by a comparator 22. A coefficient generating unit 24 generates a reverse distortion coefficient, and updates the reverse distortion coefficient stored in the reverse distortion coefficient table 14 from the comparison result of the comparator 22 such that the difference between the incoming signal before distortion compensation and the output signal becomes zero.
The pre-distorter, shown in FIG. 1, may provide satisfactory results where there are no linear errors, such as frequency amplitude deviation (linear variation of amplitude depending on frequency), group delay error (linear variation of signal delay amounts depending on frequency), and the like in the power amplifier 18. However, if there is a linear error, such as described above, in the power amplifier 18, a problem arises in that the pre-distorter that determines the reverse distortion coefficient by comparing the output signal of the power amplifier with the input signal cannot provide an accurate reverse distortion coefficient, and therefore, does not provide accurate compensation of the distortion.